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DX400 no transmit.. no preamp

C

CarlAlberts0616

New Member
Oct 31, 2019
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Hello all, first post..

I have a DX400 amplifier that is doing some weird stuff. Just want to get some opinions. Picked it up very cheap, cleaned it up and hooked everything up. Was making great power, no issues. One of the fuse holders was kinda funky so we took them out and upgraded all the power and ground wire to 8ga. Was working even better, but after a while it stopped. Here is whats going on now:

Amp on, it won't key up. Push the preamp button(which was working) and the receive goes away.
Amp off, you can talk and receive through the amp no problem.

So I replaced the keying transistor and it worked for a second, then all went away again; new transistor fried? Popped a second in to verify, worked. Stopped working, another bad keying transistor(using 2n2907a).

Don't have the box in front of me(its at work) and its driving me crazy. Going to test things tomorrow, diodes, etc. Any other ideas? Why would this transistor keep going bad? 1.5 watt dead key btw.

Thanks!
 

just a guess, but check D1, a 1N4001 diode that is in parallel with the relay coil.
It's job is to make sure that when the mic is unkeyed and the relay opens up by de-energizing the coil, that the electromotive force can't move backward into the keying circuit.
It's commonly referred to as back EMF and you will always see a diode on the coil of a relay this way, unless the relay has one built in.

if this diode were open, then you might get the symptoms you have now, where a new transistor will work for one or two key ups and then blow.

you can check the diode using a multimeter, just google how to do it.
you should remove the diode from the board before testing, or at least remove one lead first.
LC
 
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loosecannon said:
just a guess, but check D1, a 1N4001 diode that is in parallel with the relay coil.
It's job is to make sure that when the mic is unkeyed and the relay opens up by de-energizing the coil, that the electromotive force can't move backward into the keying circuit.
It's commonly referred to as back EMF and you will always see a diode on the coil of a relay this way, unless the relay has one built in.

if this diode were open, then you might get the symptoms you have now, where a new transistor will work for one or two key ups and then blow.

you can check the diode using a multimeter, just google how to do it.
you should remove the diode from the board before testing, or at least remove one lead first.
LC
Click to expand...
Thanks. Was going to start checking diodes in the morning. I am a mechanic and specialize in electrical diagnosis, but just now getting into CB and ham radio, so i am unfamiliar with these specific systems. So i do have all the toys to test stuff. Will report back in the AM!
 
CarlAlberts0616 said:
I know. Just figured I try find out what the common link between no keying up and no preamp is. Thanks.
Click to expand...

Im wondering if the relay not switching with RF input when the preamp was on cooked the preamp transistor.

My preamp didn’t work out of the box...I had the cover off for something else and dropped in a new diode and transistor, everything back to normal.

In my case I didn’t care about the preamp as I’ll never use it. It just annoys me when things don’t work.
 
dledinger said:
Im wondering if the relay not switching with RF input when the preamp was on cooked the preamp transistor.

My preamp didn’t work out of the box...I had the cover off for something else and dropped in a new diode and transistor, everything back to normal.

In my case I didn’t care about the preamp as I’ll never use it. It just annoys me when things don’t work.
Click to expand...

I also don't really care for the preamp, but I am hoping its a "clue" since it stops working when it won't key anymore. Will be heading to my office and starting to check more things out in a few here.
 
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Well, that diode was open. Replaced that and the transistor(again) and started checking all the other components. We'll see. I had other work come in and was unable to test it. REALLY bugging me. I need to set up a work desk at home! ha.
 
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That's a 1N4001 one amp diode with a very easy job across the relay coil. There is no continuous current drain heating the junction and it should not have failed. Unless, one of those problematic 10 uh chokes has been over heated and output RF is present at the diode and keying transistor.

Some other things to consider are if it burns the transistor out while keyed, that's probably due to RF and has nothing to do with inductive kickback from the relay coil. If it burns up the keying transistor when you unkey the first time, that is when the kickback would be present and could be blamed for the fault. In this case, you could key the amp as long as you wanted the first time, but only once. Also, if L1 is bad, it can place that output RF through the diode, keying transistor and preamp transistor all at once.
 
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Shockwave said:
That's a 1N4001 one amp diode with a very easy job across the relay coil. There is no continuous current drain heating the junction and it should not have failed. Unless, one of those problematic 10 uh chokes has been over heated and output RF is present at the diode and keying transistor.

Some other things to consider are if it burns the transistor out while keyed, that's probably due to RF and has nothing to do with inductive kickback from the relay coil. If it burns up the keying transistor when you unkey the first time, that is when the kickback would be present and could be blamed for the fault. In this case, you could key the amp as long as you wanted the first time, but only once. Also, if L1 is bad, it can place that output RF through the diode, keying transistor and preamp transistor all at once.
Click to expand...

Got to plug it in today. It died while keyed up. Preamp working, turned it off, keyed it up and the relay clicked on then immediately off while still keyed. Then i tried the preamp and it made recieve fade. Hm
 
Well.. This is dumb. Figured out the issue.. I think. So I noticed that the bench radio didnt have very strong receive. Normal people I talk to were pushing half what they normally do on the meter. Checked all my jumpers, no issue. Checked my SWR with my antenna hooked right in, it was sky high. Oops. It has been the same the last few years, I guess the wind storm we had did something to it last night.

So I took the box out to the truck and hooked it up with 2ga jumper cables and a good jumper just to test it out. BAM! Working great. Keys every time and its all working normal..

How dumb.. Thanks all.
 
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Shockwave said:
That's a 1N4001 one amp diode with a very easy job across the relay coil. There is no continuous current drain heating the junction and it should not have failed. Unless, one of those problematic 10 uh chokes has been over heated and output RF is present at the diode and keying transistor.

Some other things to consider are if it burns the transistor out while keyed, that's probably due to RF and has nothing to do with inductive kickback from the relay coil. If it burns up the keying transistor when you unkey the first time, that is when the kickback would be present and could be blamed for the fault. In this case, you could key the amp as long as you wanted the first time, but only once. Also, if L1 is bad, it can place that output RF through the diode, keying transistor and preamp transistor all at once.
Click to expand...

Anytime you buy a used TS amp you should pull the cover and look at the chokes that Shockwave is talking about.
If they look discolored in any way, replace them.
If they get overheated and change value/short, it is a problem.
This is a issue that all of these amps have because of the way they are designed.
If in they had spent a few bucks more this could be avoided, but......
It is what it is.
73
Jeff
 
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